“Dark pool” operations allow investors to trade large blocks of shares but keep the prices private.

Barclays has admitted misleading investors and violating securities law in the way it operated the pool. It will pay a $70m fine.

Credit Suisse will pay $60m and another $24.3m relating to other violations.

The fines will be split between the State of New York and the Securities and Exchange Commission (SEC).

The New York Attorney General and the SEC have both censured the two banks for their misconduct.

Ralph Silva, banking analyst from SRN, told BBC News: “The fines are a message, not a punishment. The levels are insignificant compared to the profits in this line of business.”

“Regulators are telling the banks to close the vulnerabilities, something the banks have been reluctant to do because answers come with high operational price tags.”

Thorsten Beck, professor of banking and finance at Cass Business School, said the fines “might make the banks more reluctant to invest in this specific type of trading, but might just divert their appetite towards other money-making activities”.

“I am a bit critical of these fines as they seem rather ad-hoc without looking at the bigger question of what banks should, and should not, be doing.”

‘Significant role’

New York Attorney General Eric Schneiderman said: “These cases mark the first major victory in the fight against fraud in dark pool trading that began when we first sued Barclays.”

He added that “co-ordinated and aggressive government action” had led to “admissions of wrongdoing, and meaningful reforms to protect investors from predatory, high-frequency traders”.

“We will continue to take the fight to those who aim to rig the system and those who look the other way.”

Meanwhile Andrew Ceresney, director of the SEC’s enforcement division said: “Dark pools have a significant role in today’s equity marketplace, and the firms that run these venues must ensure that they do not make mis-statements to subscribers about their material operations.”

Credit Suisse will neither admit nor deny the allegations.

A spokeswoman said the bank was “pleased to have resolved these matters” with the SEC and the New York attorney general.

A Barclays representative said: “the agreement will enable us to focus all of our efforts on serving our clients”.